Our analysis of calcium current inactivation in rat intermediate lobe cells showed that the amplitude of the calcium current in these cells is strongly dependent on the prior history of the membrane potential. The calcium channel inactivates at a very slow rate that has not been described previously. This finding is of biological importance since it indicates that secretion may be modulated in these cells simply by regulating the cell resting membrane potential. A more detailed understanding of these slow changes in calcium channel activity will be examined by analyzing channel inactivation at the single channel level. Single calcium channels will be characterized on the intermediate lobe cells in tissue culture. The changes of channel kinetics with prolonged depolarizations will be recorded and the changes in channel open probability will be compared to the previously published time course of calcium current inactivation.